25 Jan 2023
25 Jan 2023
Status: this preprint is currently under review for the journal ACP.

Particle shapes and infrared extinction spectra of nitric acid dihydrate crystals: Optical constants of the β-NAD modification

Robert Wagner1, Alexander D. James2, Victoria L. Frankland3, Ottmar Möhler1, Benjamin J. Murray4, John M. C. Plane2, Harald Saathoff1, Ralf Weigel5, and Martin Schnaiter1 Robert Wagner et al.
  • 1Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology, Karlsruhe, Germany
  • 2School of Chemistry, University of Leeds, Leeds, UK
  • 3School of Chemistry and Chemical Engineering, University of Surrey, Guildford, UK
  • 4School of Earth and Environment, University of Leeds, Leeds, UK
  • 5Institute for Atmospheric Physics, Johannes Gutenberg Universität Mainz, Mainz, Germany

Abstract. Satellite- and aircraft-based mid-infrared measurements of polar stratospheric clouds (PSCs) have provided spectroscopic evidence for the presence of β-NAT (nitric acid trihydrate) particles. Metastable nitric acid hydrate phases such as α-NAT and α-NAD (nitric acid dihydrate) have been frequently observed in laboratory experiments, but not yet detected as a constituent of PSCs in atmospheric measurements. As for the β-NAD modification, its formation was first observed in X-ray diffraction measurements when the low-temperature α-NAD phase was warmed to a temperature above 210 K. Its infrared spectrum has been reported, but so far no optical constants have been derived that could be used as input for infrared retrievals of PSC composition. In this work, we show that β-NAD particles were efficiently formed in isothermal, heterogeneous crystallisation experiments at 190 K from supercooled HNO3/H2O solution droplets containing an embedded mineral dust or meteoric smoke particle analogue. An inversion algorithm based on a T-matrix optical model was used to derive for the first time the mid-infrared complex refractive indices of the β-NAD modification from the measured extinction spectrum of the particles. In contrast to the heterogeneous crystallisation experiments, the α-NAD phase was formed when the HNO3/H2O solution droplets did not contain a solid nucleus and crystallised homogeneously. Using a light scattering detector that recorded two-dimensional scattering patterns of the crystallised NAD particles, we were able to determine predominant shapes of the α- and β-NAD crystals. We found that α-NAD grew into elongated, needle-shaped crystals, while β-NAD particles were compact in shape. This agrees with previously reported images of α- and β-NAD particles grown on the cryo-stage of an Environmental Scanning Electron Microscope.

Robert Wagner et al.

Status: open (until 08 Mar 2023)

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Robert Wagner et al.

Robert Wagner et al.


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Short summary
Polar stratospheric clouds (PSCs) play an important role in the depletion of stratospheric ozone. They can consist of different chemical species, including crystalline nitric acid hydrates. Infrared measurements are an important technique for determining the composition, shape and phase of PSC particles. For the quantitative analysis of infrared spectra, we have for the first time determined accurate optical constants and predominant particle shapes of nitric acid dihydrate in its β phase.